Subtyping polymorphism!

polysub.js

// @flow
type Foo<A> = { n: number, extra: A };
type Foo2 = Foo<{s: string}>;

type Bar<A> = { foo: Foo<A> };
type Bar2 = { foo: Foo2 };

function getBarN(bar: Bar){ return bar.foo.n }
function getBarS(bar: Bar2){ return bar.foo.extra.s }

let bar2: Bar2 = { foo: { n: 2, extra: {s: ""} }};
getBarN(bar2);
getBarS(bar2);